1. Field of the Invention
This invention relates to organopolysiloxane compositions. More particularly, this invention relates to the use of extrudable compositions to prepare silicone elastomers exhibiting a desirable combination of high durometer hardness, tensile strength and tear strength following exposure to temperatures above 150.degree. C. for extended periods of time.
2. Description of the Prior Art
Silicone elastomers, a term which includes elastomeric organopolysiloxanes, can be prepared using a variety of organosiloxane oligomers and polymers, fillers and curing mechanisms. The choice of a particular combination of organosiloxane, filler, curing agent and reaction conditions is governed at least in part, by the physical properties desired in the cured elastomer.
The formulations employed to prepare elastomeric organopolysiloxanes range in viscosity from pourable liquids to non-flowable gums which can be processed only under the high level of shear achieved using a two- or three-roll rubber mill.
The prior art discloses elastomeric polyorganosiloxanes exhibiting various combinations of properties desired for particular end use applications, however elastomers exhibiting certain desirable combinations of properties, such as high values of hardness and tear strength that are not adversely affected by heat, have not heretofore been described. The present invention concerns elastomeric polyorganosiloxanes exhibiting such a desirable combination of properties and extrudable formulations for preparing these products.
A polyorganosiloxane composition that is curable by peroxides to yield elastomers exhibiting a high level of tensile strength at elevated temperatures is disclosed in U.S. Pat. No. 3,122,516 to Polmanteer. The desired property is achieved using a reinforcing silica filler containing certain vinyl substituted silyl units in combination with other non-vinyl containing silyl units. Specified concentrations of these units are attached through siloxane linkages to the silicon atoms on the surface of the filler particles. The modified silica filler defined by Polmanteer contains 50 to 100 mol percent SiO.sub.2 units (W) and 0 to 50 mol percent RSiO.sub.1.5 units (X). The surfaces of the filler particles are saturated with EQU R.sub.n SiO.sub.4-n/2
units (Y) and EQU R.sub.x (CH.sub.2 .dbd.CH)SiO.sub.3-x/2
units (Z) where there are 4 to 29.9 (Y) units and 0.1 to 2 (Z) units per 100 (W) and (X) units and a total of 6 to 30 (Y) and (Z) units per 100 (W) and (X) units. In the foregoing formula, n has an average value from 2 to 3 and the average value of x is from 1 to 2. Polmanteer further suggests that the best results are obtained using organopolysiloxanes having 99 to 99.5 inclusive mol percent dimethylsiloxane units and 0.05 to 1 inclusive mol percent methylvinylsiloxane units. From 20 to 100 parts of filler per 100 parts of polysiloxane gum must be used to achieve the improved hot strength, where the viscosity of the polymer and the desired amount of filler are in an inverse ratio to each other. However, Polmanteer does not suggest compositions of this invention which are readily extrudable and cure to yield elastomers exhibiting high tear strength and high durometer hardness following exposure to temperatures above 150.degree. C. for extended periods of time.
Polmanteer et al. in U.S. Pat. No. 3,697,473 describe a composition which is flowable at ambient temperature. The composition consists essentially of (1) a polydiorganosiloxane having a vinyl group in each of the two terminal triorganosiloxy groups, and (2) a mixture of silicon compounds containing silicon-bonded hydrogen atoms, one compound containing two silicon-bonded hydrogen atoms per molecule and another compound containing 3 to 10 silicon-bonded hydrogen atoms per molecule. The composition is cured using a platinum catalyst and, optionally, a reinforcing silica. The compositions described by Polmanteer et al. are fluid and the cured elastomers have high tensile strengths and elongations. Such compositions may be useful for many applications, but do not achieve the combination of high durometer hardness with high tensile and tear strengths required for certain applications.
In U.S. Pat. No. 3,884,866, Jeram et al. describe an organopolysiloxane composition suitable for low pressure injection molding. The composition yields elastomers having high tensile strength, good elongation but relatively low tear strength. These compositions described by Jeram et al. comprise 20 to 90 parts of a vinyl-containing organopolysiloxane having a viscosity of 5,000 to 1,000,000 centipoises, 5 to 40 parts of a vinyl-containing organopolysiloxane having a viscosity of 50 to 5,000 centipoise, 5 to 70 parts of filler and 0.1 to 50 parts per million of a platinum catalyst. This composition is cured by mixing 100 parts of it with 1 to 100 parts of a composition containing silicon-bonded hydrogen atoms. These Jeram et al. compositions appear to buffer the same problems encountered with other prior art compositions. Jeram et al. teach that higher durometer hardness values can be achieved by increasing the concentration of a treated silica filler, however the additional filler increases the viscosity of the composition, making it more difficult to process.
U.S. Pat. No. 4,162,243 to Lee, Maxson and Stebleton (hereinafter referred to as Lee) teaches using a liquid vinyl endblocked polydimethylsiloxane fluid wherein the molecular weight distributions of the various polymer species that constitute the fluid are within specified ranges. Formulations containing these fluids are extrudable under relatively low pressures and can be cured to elastomers exhibiting durometer hardness values of up to 65 on the Shore A scale in combination with a tensile strength of 6.97 megapascals and a tear strength of 35.0 kilonewtons per meter. While these property levels may be adequate for certain end use applications, other applications require even higher levels of durometer hardness and tear strength following exposure of the elastomer to temperatures above 150.degree. C. for extended periods of time. While certain of the control formulations employed by Lee for comparative purposes exhibit durometer hardness values as high as 68, the tear strength of these products is only 5.08 kilonewtons per meter, compared with 35.0 kilonewtons per meter for elastomers prepared in accordance with the teaching of Lee.
It has now been found that the levels of durometer hardness and tear strength achieved using the type of polyorganosiloxane formulations disclosed by Lee can be substantially increased by introducing into these formulations from 20 to about 30% by weight, based on the weight of vinyl endblocked polydimethylsiloxanes, of certain vinyl-containing polysiloxane resins.
Polyorganosiloxane elastomer formulations employing a polyorganosiloxane resin containing vinyl groups are disclosed in U.S. Pat. No. 4,163,081, which teaches that the resins can be optionally added to improve the adhesion of polyorganosiloxanes, particularly elastomers, to substrates such as glass, aluminum and stainless steel.
U.S. Pat. No. 4,123,604 teaches employing vinyl-containing resinous copolymers containing R.sub.3 SiO.sub.1/2, R.sub.2 (CH.sub.2 .dbd.CH--SiO.sub.1/2 and SiO.sub.4/2 units in combination with specified rhodium catalysts and a polydiorganosiloxane to obtain a coating material exhibiting a desirable range of release values when applied to a paper substrate. It would not be obvious from the disclosures in this patent to add a carefully controlled amount of a vinyl-containing organopolysiloxane resin to an elastomer formulation containing a vinyl endblocked polydiorganosiloxane and a silica filler containing a specified concentration of vinyl groups on the surface of the silica particles with the objective of increasing the durometer hardness of the final elastomer without adversely affecting other properties such as tensile strength, elongation and tear strength.
The optional use of certain vinyl-containing organopolysiloxane resins to improve the mechanical strength of elastomeric organopolysiloxanes is suggested in U.S. Pat. No. 4,077,943, however this reference does not specify which properties can be improved or the advantages achieved by combining the vinyl-containing resin with a silica filler containing a small concentration of vinyl groups bonded to the surface of the silica particles.
U.S. Pat. No. 3,699,073 to Wada et al. discloses organopolysiloxane elastomers prepared from liquid compositions containing a vinyl endblocked polydiorganosiloxane, an optional organopolysiloxane resin containing SiO.sub.4/2, R.sub.3 SiO.sub.1/2 and CH.sub.2 .dbd.CH(R.sub.2)SiO.sub.1/2 groups, a relatively low molecular weight organosiloxane copolymer containing chain terminating groups of the formula R"R.sub.2 SiO.sub.1/2, R.sub.2 (R'O)SiO.sub.1/2 or R.sub.2 (HO)SiO.sub.1/2 and an inorganic filler. The disclosure of Wada et al. is sufficiently broad to encompass all but one of the classes of ingredients present in the formulations employed in the method of the present invention, however this disclosure also encompases formulations that will not yield elastomers exhibiting the unique combination of heat stability, high durometer hardness, high tensile strength and high tear strength which distinguishes the products of the present invention from prior art materials, including those exemplified in the aforementioned Wada et al. patent. Nowhere in this patent is there any teaching concerning (1) a requirement for a critical selection of specific types and relative amounts of ingredients from the broad ranges disclosed in the patent or (2 ) the unique combination of desirable properties that can be achieved by adhering to such a requirement.